What is the difference between SGA and PGA in Oracle?
If the SGA is the "communal kitchen" where everyone shares the same ingredients and tools, the PGA is the "private lunchbox" each worker brings to the table.
In Oracle Database architecture, managing memory efficiently is the secret sauce to performance. To do that, Oracle splits memory into two distinct areas: the System Global Area (SGA) and the Program Global Area (PGA).
The SGA is a shared memory region. It is allocated when the Oracle Instance starts and is shared by all background and server processes. Its primary goal is to minimize disk I/O by keeping frequently used data in RAM.
Database Buffer Cache: Stores the actual data blocks. If two different users want to see the "Employees" table, they both look here.
Shared Pool: Stores the "plans" for SQL queries. If User A runs a complex query, Oracle remembers how it did it so that when User B runs the same query, it doesn't have to figure it out again.
Redo Log Buffer: A temporary "waiting room" for data changes before they are written to the physical Redo Logs on disk.
The PGA is a non-shared memory region. It contains data and control information specifically for a single Oracle server process. Every time a user connects to the database, a new PGA is created just for that session.
SQL Work Areas: Used for memory-intensive operations like Sorting (ORDER BY) or Hash Joins.
Session Memory: Stores user-specific information like login credentials and session variables.
Private SQL Area: Stores information about bind variables and the current state of a cursor for that specific user.
| Feature | SGA (System Global Area) | PGA (Program Global Area) |
| Accessibility | Shared by all processes. | Private to one specific process. |
| Allocation | Allocated at Instance startup. | Allocated when a session connects. |
| Purpose | Data caching and shared SQL plans. | Sorting, joining, and session variables. |
| De-allocation | Only when the Instance shuts down. | When the user session ends. |
| Size Management | SGA_TARGET parameter. | PGA_AGGREGATE_TARGET parameter. |
Imagine you are running a massive report that sorts 10 million rows of data.
That sorting happens in your PGA.
If your PGA is too small, Oracle has to "spill" that sort to the disk (Temporary Tablespace), which is much slower.
However, the data being sorted was originally read from the SGA (Buffer Cache).
By separating these two, Oracle ensures that one user’s massive "Sort" operation doesn't wipe out the "Data Cache" that everyone else is using to stay fast.
In modern Oracle versions, you don't have to micro-manage these individually. You can use AMM (Automatic Memory Management) by setting the MEMORY_TARGET parameter. Oracle will then dynamically move memory between the SGA and PGA based on the database's workload!